1. Field of the Invention
The present invention relates to an illumination optical system and an image projection apparatus.
2. Description of the Related Art
Conventionally, there have been used illumination optical systems which include a fly-eye lens and a polarization conversion element to uniformly and efficiently illuminate an image display element such as a liquid crystal light valve. In order to increase illumination efficiency in the illumination optical system, an amount of the light beam passing through the effective region formed by the lens cell of the fly-eye lens and the polarization conversion element needs to be increased.
Light source images formed by the illumination optical system are each in a rod shape, which reflects the shape of a discharge arc that is the light source, and are formed to have an inclination according to the position on the reflective surface of a paraboloid reflector through which the light beam passes to form the light source image. Thus, the light source images have a radial distribution around an optical axis of the illumination optical system. Meanwhile, the effective regions formed by the fly-eye lens and the polarization conversion element are rectangular. Thus, the light source image may stick out of (project from) the effective region, resulting in a reduction in the illumination efficiency of the illumination optical system.
For the light source images that spread out radially because the paraboloid reflector is used, the effective region is preferably square in order to obtain maximum illumination efficiency with the effective region of a minimum area. Japanese Patent Laid-open No. H07-181392 discloses a configuration where the effective region is made square to maintain the illumination efficiency.
In the configuration of Japanese Patent Laid-open No. H07-181392, however, in order to further increase light use efficiency, for example a polarization conversion element needs to be arranged. However, the width of a polarization beam splitter constituting the polarization conversion element is half the width of the lens cell of the fly-eye lens. Therefore, the effective region is in a rectangular shape, resulting in a loss being likely to occur.